High post-irradiation ductility process

ABSTRACT

A method of treating certain stainless steels to improve the ductility characteristics after irradiation at high temperatures involving working at room temperatures or lower to a 90 percent reduction of area followed by low temperature recrystallization annealing.

United States Patent 1191 Chow June 19, 1973 HIGH POST-IRRADIATIONnucrrurv 3,573,109 3/1911 Levy .Q ..14s/12.3

OC 3,623,920 11/1971 Kondo et a]. 148/123 Inventor: Joe G. Y. Chow,Northport, N.Y.

Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission, Washington, DC.

[22] Filed: Apr. 20, 1972 [21] Appl. No.: 246,046

[52] 0.8. CI. 148/12, 148/123 [51] Int. Cl. C2ld 1/00, C2ld 7/02 [58]Field of Search..... 148/123, 12

[56] References Cited UNITED STATES PATENTS 3,347,715 10/1967 Pfeil148/12 Kondo et al. 148/123 Primary ExaminerW. W. StallardAttorneyRoland A. Anderson [57] ABSTRACT A method of treating certainstainless steels to improve the ductility characteristics afterirradiation at high temperatures involving working at room temperaturesor lower to a 90 percent reduction of area followed by low temperaturerecrystallization annealing.

7 Claims, 2 Drawing Figures HIGH POST-IRRADIATION DUCTILITY PROCESSBACKGROUND OF THE INVENTION The development of breeder nuclear reactorswhich will operate at high temperatures for long periods of time betweenshutdowns requires the use of materials which will not become embrittledand crack under the adverse conditions of temperature and intenseradiation. For example, it is known that the effect of irradiation onstainless steel at temperatures of about 1000 F. is normally to reducesubstantially the ducility of the metal. The problem of cracking inservice is one of the more severe difficulties faced by breeder reactordesigners.

In US. Pat. No. 3,620,252 there is described a process for treatingcobalt alloys to improve their ducility under the described conditions.However, as cobalt during reactor service produces radioactive cobalt 60which has a very long half life as a gamma source and some of it tendsto circulate with the coolant, the resulting maintenance problems makeit preferable to avoid the cobalt alloys.

SUMMARY OF PRESENT INVENTION In accordance with this invention, it hasbeen found that chromium nickel stainless steel alloys, which areotherwise suitable for reactor use and breeder reactors in particular,can be improved in their ability to remain ductile after being subjectto irradiation at high temperatures for long periods of time. Bychromium-nickel stainless steel alloys is meant for the purposes of thisinvention an austenitic alloy of iron having as its major alloyingingredients chromium and nickel, with chromium as the major additive andnickel as the minor additive, and have no more than 2 percent manganese.More detailed information on such stainless steel alloys including AISIdesignated types are given in the Metals Handbook, 8th E., Vol. 1, pub.by American Society of Metals, pp. 408- 409. I

In accordance with this invention, a stainless steel semifinished sampleor article is cold worked until the material is too hard to work,usually when the cross section area is reduced at least 50 percent, isthen given an intermediate low temperature recrystallization, is coldworked again until the maximum dimension of substantially all grains inthe sample do not exceed 3 microns, the cross section area being reducedat least 90 percent, and is then given a final annealing atrecrystallization temperatures to produce long term stability.

It is thus a principal object of this invention to provide an improvedprocess for treating stainless steel a1- loys to retain ductility duringand after irradiation.

Other objects of this invention will hereinafter become obvious from thefollowing description of preferred embodiments of this invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a photo-micrograph (1000 X)of a section of normal type 316 stainless steel after beingsubject toirradiation.

FIG. 2 is a photo-micrograph (1000 X) of similar material which had beenfirst treated in accordance with a preferred embodiment of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with a preferredembodiment of this invention a cylindrical rod of a chromium-nickelstainless steel alloy at a temperature up to 300 F, but preferably at orbelow room temperature is cold worked, such as by either swaging orrolling, reducing the cross section by at least 50 percent or until itbecomes too hard to work, is given an intermediate low temperatureanneal by heating it to at least recrystallization temperature, in therange of 1200-1500 F. until it is soft enough to resume working, afterwhich working is resumed until the maximum dimension of substantially(that is, at least percent) all grains in the alloy is not in excess of3 microns, and then low temperature annealing is repeated for 10-16hours or a sufficient length of time to produce long term stability ofthe grain structure. The final annealing is for asubstantially longertime than the intermediate annealing.

More than one intermediate annealing step'may be employed, or in somecases, it may bepossible to produce the 90 percent reduction in areawithout an intermediate annealing. Multiple annealings as describedinsure that an extremely fine-grain structure results, in

which the maximum dimension of substantially all grains does not exceed3 microns. The following examples illustrate this invention;

Example 1 A 1 inch cylindrical sample ofType 316 stainless steel wascold swaged at room temperature to a diameter reduced by 45 percent andrecrystallized at 870 C.

for 16 hours.

The worked sample was then irradiated to 1.3 X 10 nvt (E 0.82 MeV) andspecimens were prepared and tested for ductility at several temperatureswith the results shown in Table Az TABLE A Test Tamp., C TensileStrength, psi. Total E1ong.,% 25 148,000 10 650 42,800 25 750 29,500 7800 17,800 6.5

FIG. 1 is a photomicrograph of the microstructure prior to irradiation.

Example 2 TABLE B Yield Tensile Total Test Temp. Strength StrengthElong. C. psi psi 25 100,000 121 .000 23 650 26,000 35,100 32 750 12,80015,200 35 800 10,600 12,000 30 FIG. 2 is a photomicrograph of themicrostructure of the specimen prior to irradiation showing the smallgrain size.

A comparison of the results shown in'Tables A and B shows a remarkableincrease in ductility of the stainless steel when the present inventiveprocess is applied.

Examples 3 and 4 of substantially all grains do not exceed 3 microns;

and

d. annealing said sample at a temperature at or above recrystallizationto stabilize the crystal structure.

2. The process according to claim 1 in which working takes place up to atemperature of 300F.

3. The process according to claim 2 in which the intermediate annealingtemperature is in the range of l200-l500 F.

4. The process according to claim 3 in which final annealing takes placeat a temperature in the range of l200l500F. until recrystallizationtakes place.

5. The process according to claim 4 in which the working temperature isroom temperature.

6. The process according to claim 4 in which more than one intermediateannealing is employed.

7. The process according to claim 3 in which the final annealing is fora substantially longer period than the intermediate anneal to increaselong term stabilization characteristics of the grain structure.

2. The process according to claim 1 in which working takes place up to atemperature of 300*F.
 3. The process according to claim 2 in which theintermediate annealing temperature is in the range of 1200*-1500* F. 4.The process according to claim 3 in which final annealing takes place ata temperature in the range of 1200*-1500*F. until recrystallizationtakes place.
 5. The process according to claim 4 in which the workingtemperature is room temperature.
 6. The process according to claim 4 inwhich more than one intermediate annealing is employed.
 7. The processaccording to claim 3 in which the final annealing is for a substantiallylonger period than the intermediate anneal to increase long termstabilization characteristics of the grain structure.